1. Field of the Invention
The present invention relates to a method and an apparatus for arranging balls on a mounting object. More particularly, the present invention relates to a method and an apparatus for arranging fine balls serving as ball bumps on an electrode on a semiconductor chip, an electrode on a semiconductor-mounting substrate, or on a semiconductor device electrode.
2. Description of the Related Art
Fine balls (bumps) are used as a medium for connecting an electrode on a semiconductor chip substrate and an external circuit. The technique of forming bumps for a ball grid array (BGA) by transferring fine balls onto a semiconductor substrate or a semiconductor chip is becoming more important. In this specification, a semiconductor chip for transferring balls and the like are collectively referred to as a mounting object.
The conventional technique for transferring balls onto a mounting object is as follows. A ball arrangement device has a ball suction holes pierced for sucking many balls. The positions of the ball suction holes correspond to positions at which the balls on a semiconductor chip or the like are to be transferred. All the balls are previously sucked in the suction holes, and the ball arrangement device is transferred to a mounting base. The mounting object onto which the balls are to be transferred is placed on the mounting base in advance. The balls can be transferred by pressing the balls sucked on the ball arrangement device onto the surface of the mounting object on the mounting base.
Along with the recent higher integration of semiconductor devices, there is an increasing number of electrodes for a single semiconductor. Usually, a semiconductor is provided with 100 to 300 electrodes, and more recently, a single semiconductor device has even 500 to 1,000 electrodes. As a result, the area of a semiconductor tends to increase, and the size of a ball is becoming smaller, and at the same time, there is a tendency of the arrangement interval between balls toward a narrower value. Furthermore, in the aforementioned technique of transferring the balls as a bump, it is becoming a common practice not only to transfer the balls in a number equal to that of electrodes for a semiconductor in a lump, but also to mount balls in a lump on a plurality of chips, thereby improving productivity.
In the above-mentioned conventional technique, many fine balls having a uniform size are provided in a tray. The surface sucking the balls of the ball arrangement device is brought closer to the fine balls in the tray. The ball suction holes are caused to suck the fine balls by sucking air through reduction of pressure on the back of the ball suction holes (FIG. 18). Suction can more easily be accomplished by vibrating the tray upon suction to cause the ball to jump in the tray, or by causing the balls to float in the tray by air blow through the bottom of the tray.
The fine balls fed into the tray may contain balls having a diameter off the standard upper limit or off the standard lower limit mixed therein although in a slight amount. While balls having a larger diameter off the standard upper limit can easily be excluded in the manufacturing stage of balls, exclusion of balls having a smaller diameter off the standard lower limit may sometimes be insufficient. In the conventional ball arrangement apparatus, which does not have a function of excluding defective balls having a diameter smaller than the standard lower limit, mixture of defective balls would result in mounting of balls of an insufficient diameter on a product such as a ball grid array. When using solder balls, it is the usual practice to preliminarily attach the solder balls to the mounting object with bonding power of flux. small diameter of solder balls results in occurrence of such defects as some of solder balls left uncoated with flux or solder balls not coming into contact with the electrodes. When using gold balls, means for thermally pressing the gold ball against the object is employed. Gold balls having a small diameter lead to a defect in that sufficient pressing is unavailable, making it impossible to obtain pressure contact.
Because of feeding balls in a large quantity to the tray to cause the balls to jump therein, and as required to vibrate the tray, dust and dirt are inevitably entangled into the tray from surroundings. Since dirt and dust are mixed with the balls in a large quantity in the tray, it is impossible to remove them. If dust is sucked into the ball suction hole of the ball arrangement device in place of the balls, arrangement of the balls ends in failure.
In order to remove balls in excess adhering to the ball arrangement device, there are proposed a method for excluding balls in excess with an air blow or a brush after suction of the balls, and a method, disclosed in Japanese Unexamined Patent Publication No. 8-64,944(U.S. Pat. No. 5,657,528), of causing relative displacement of the ball arrangement device and the tray of the balls.
It has been the conventional practice to adopt a diameter of the ball suction hole of the ball arrangement device about 0.5 times as large as the diameter of the balls to e sucked. With a larger diameter of the ball suction hole, the balls cut into the ball suction holes under the pressure applied upon transfer of the balls onto the mounting object. When transferring gold balls requiring adoption of thermal pressure-contact, pressure is high, and when transferring solder balls, not requiring a high transferring pressure, the solder balls are susceptible easy deformation: it is therefore necessary to use a smaller diameter of the ball suction holes to prevent cut-in of the balls in all cases. As the sucking force of the ball is proportional to the cross-sectional area of the suction hole, a smaller diameter of the suction hole inevitably leads to a smaller sucking force.
When using solder balls, flux is usually transferred to the position of transfer on the mounting object or to the solder balls prior to transferring the balls to the mounting object. Transfer carried out without taking notice of adherence of excessive balls may a trouble of the flux adhering via the balls to the ball arrangement device. This not only results in a single defective semiconductor device to which excessive balls have been transferred, but also makes it impossible to start the next operation unless the ball arrangement device having the flux adhering thereto is washed, thus causing a serious decrease in equipment productivity. It is therefore necessary to prevent occurrence of an abnormality of adhering balls in excess.
A first object of the present invention is to exclude balls of a small diameter off the standard lower limit and entangled dust and dirt before sucking the balls into the ball arrangement device, thereby preventing failure of arrangement of the balls.
In the conventional method of sucking the balls jumping or floating in the ball tray into the ball suction holes of the ball arrangement device, each ball sucking hole must suck one ball. However, a plurality of balls may be sucked into a single ball suction hole, or balls may adhere to a portion other than the ball suction hole on the ball arrangement device, or there may be some ball suction holes not sucking the balls.
An attracting force may act between a ball and the ball arrangement device, so that a ball may be attracted at a portion other than the portions where vacuum attracted acts as at the ball suction holes. In order to exclude these excessive balls by means of an air blow or a brush, it is necessary to impart a force of a prescribed magnitude to these balls in excess. On the other hand, the suction force of the balls to the ball suction holes is proportional to the cross-sectional area of the suction hole and the suction pressure. Because the diameter of the ball suction hole is only about 0.5 times as large as the diameter of the ball as described above, the sucking force of the ball to the suction hole is not sufficient. If an air blow or brush cleaning with a force sufficient to exclude the excessive balls, therefore, even balls having been sucked properly in the ball suction holes are excluded.
Under the aforementioned circumstances, in the conventional ball arrangement method in which the occurring ratio of adhering abnormality of excessive balls is high, and the detecting ability of adhering abnormality of excessive balls is not sufficient, it has been difficult to eliminate a trouble in which an abnormality was not detected by detecting means and balls were transferred as they were onto the mounting object.
In the conventional method of sucking balls jumping or floating in the tray into the ball suction holes of the ball arrangement device, a problem has been encountered in which suction of many balls in a lump took a very long period of time before completion of suction into all the ball suction holes. Particularly, when sucking 1,000 or more balls, suction has required a long time of from 8 to 15 seconds.
A second object of the present invention is to provide ball arrangement method and apparatus which permits prevention of occurrence of a suction abnormality such as an excessive suction or insufficient suction, and reduction of time for suction of balls.
It is possible to effect normal transfer of balls onto the mounting object by sucking one ball into each of all the ball suction holes of the ball arrangement device. Transfer of the balls onto the mounting object while some suction holes do not contain sucked balls results in a defective mounting object such as a semiconductor chip because of shortage of balls. After suction of the balls into the ball suction holes of the ball arrangement device, therefore, it is the usual practice to conduct an inspection to discover a ball suction abnormality prior to transferring the balls onto the mounting object. The inspection is generally carried out by the use of optical detecting means. For example, generally known methods include a method of observing from front of the ball arrangement device by means of an ITV; and a method of detecting the presence of a light passing through the ball suction holes as disclosed in Japanese Unexamined Patent Publication No. 8-153,960(U.S. Pat. No. 5,601,229). When a ball suction abnormality is discovered in the inspection, the balls are not transferred onto the mounting object, but suction to the ball arrangement device is tried again, thereby preventing occurrence of a trouble in which a defective semiconductor chip which is the mounting object results.
When carrying out an inspection for discovering a ball suction abnormality by the method using an ITV, it is necessary to pick up an image of a condition of suction with an image pick-up apparatus, process the thus taken image by an image processor, and further, provide detecting means for determining the presence of a ball suction abnormality from the processed image. Upon shooting the image, presence of individual balls must be recorded within the range of resolution of the shooting apparatus. Because there is a tendency of the area for mounting in a lump becoming larger, the size of the ball becoming smaller, and the interval between arranged balls becoming smaller, it may sometimes be impossible to shoot and inspect the area for arrangement in a lump at a time. In such a case, a considerably increased inspection time results in a longer tact time of ball arrangement, after causing impairment of productivity. Another problem is that it is necessary to provide a shooting apparatus, an image processor and determining and detecting means, leading to an increase in the inspection equipment cost. The position of ball arrangement and the member of balls are basically different for each kind of mounting object, and it is necessary to individually build suction abnormality determining/detecting means for each kind.
In the method of detecting the presence of a light passing through the ball suction holes, it is necessary to detect a leakage light by gathering beams from passing light channels in a number equal to the member of balls to be sucked. This results in a problem of the inner structure of the ball arrangement device becoming larger in scale, which in turn leads to a lower operating speed of the apparatus and an increase in manufacturing cost.
A third object of the present invention is to provide ball arrangement method and apparatus having detecting means which permits detection of a ball suction abnormality without fail in a simple manner.